1. Field of the Invention
The present invention relates to a light beam scanner unit for forming a single electrostatic latent image on a single photosensitive drum by scanning the photosensitive drum with plural laser light beams simultaneously, and an image forming apparatus using this light beam scanner unit such as, for instance, digital copying machines, laser printers, etc.
2. Description of the Related Art
In recent years various kinds of digital copying machines have been developed for forming images through scanning and exposing with laser beams and electro-photographic process.
Recently, in order to achieve high speed image formation, a digital copying machine has been developed.
This digital copying machine employs a multi-beam system, that is, a number of laser beams are generated and plural lines are scanned simultaneously with these plural laser beams.
This multi-beam system digital coping machine is equipped with an optical system unit, which is a light beam scanner unit, comprising mainly plural semiconductor laser oscillators for generating laser beams, a rotary polyhedral mirror such as polygon mirror, which reflects laser beams that are output from these plural laser oscillators toward a photosensitive drum and scans the photosensitive drum with the laser beams, a collimator lens and f-.theta. lens.
Further, in order to obtain an image of the high quality using the multi-beam optical system, the light beam powers are normally controlled so as to make them equal on the photosensitive drum.
In this light beam power control, light beams that are led onto a photosensitive drum by a polygon mirror are sensed with a light sensor element and converted into current corresponding to quantity of light, the current that is output from this light sensor element is converted into voltage in a current-voltage converter, the voltage output from this current-voltage converter is integrated in an integrator, the output of this integrator is converted into a digital signal by an A/D converter and power sensor information is obtained, and laser oscillators are controlled according to this power sensor information so that light beam powers for scanning the photosensitive drum become a fixed value.
Further, in a digital copying machine of multi-beam system, a light beam passing position in the sub-scanning direction is controlled in order to form an image of high quality.
In this light beam passing position control, the passing position of light beams led on a photosensitive drum by a polygon mirror in a direction orthogonal to the scanning direction of the light beams is sensed with a light bean sensor element, an optical path deflection volume to deflect the light beam passing position to a desired position on a photosensitive drum is calculated based on this sensing result and based on this calculated optical path deflection volume, an optical path deflection means such as galvanomirrors, etc. are controlled and thus, the deflection of the light beam passing position on a photosensitive drum is controlled.
However, in a conventional light beam power control, such a problem was caused as shown below.
That is, the polygon mirror is normally rotating at a high speed of about 20,000 [rpm] and the surface of the polygon mirror is contaminated by dust in the air and silicone generated from a fixing unit in an image forming apparatus in many cases.
In particular, the contamination of the edge portion at the top of the rotating direction of the mirror is remarkable.
FIG. 19 shows the state of contamination on the mirror surface. Reference numerals 35 and Q show the contamination on the polygon mirror and the mirror surface, respectively. The area near this edge portion is a portion where light beams are reflected when passing over a light beam power sensor element, and the contamination Q of this portion largely affects the light beam sensor characteristic.
That is, for instance, when the area near the central portion of the light receiving surface of light beam power sensing element is scanned with light beams by the polygon mirror, the light beams are partially reflected irregularly on the polygon mirror due to the contamination of the polygon mirror surface, changing the light beam. When this light beams are focussed on the light receiving surface of the light beam power sensing element, quantity of light beams becomes less than that when the mirror surface is not contaminated (quantity of light reaching the light receiving surface decreases).
In this case, because the output of the light beam power sensing element becomes sharply smaller than at the normal state, a light emitting power indicating value to laser oscillators is increased so that the output of the light sensing element becomes a fixed value. Thus, on the light receiving surface of the light beam power sensing element, laser power at the normal state is obtained even when the polygon mirror surface is contaminated. However, as the polygon mirror surface for forming an image is not contaminated, an image forming unit forms an image with a light beam power larger than that at the normal state. As a result, collapsed portions are caused on an output image and the quality of image becomes worse.
Further, the contamination on the polygon mirror surface expands up to the central portion of the polygon mirror with the lapse of time. In this case, the light beam shape changes on the image forming unit and the quality of image is deteriorated.
Further, when the polygon mirror surface is contaminated remarkably, quantity of light minimum required for the light beam power sensing element as well as the light beam passing position sensing element to operate cannot be secured and it is considered that even the light beam passing position control cannot be made. In this case, as a matter of course, the quality of image becomes worse.